This invention relates to the field of magnetic resonance imaging (MRI) utilizing nuclear magnetic resonance (NMR) phenomena. It is particularly related to novel apparatus and method for acquiring NMR images from adjacent planar volumes or slices of a three-dimensional subject while reducing non-imaged "gaps" between such adjacent slices--while yet permitting the necessary NMR image data to be obtained in a single complete measurement sequence. This advantageous result simultaneously permits some relaxation in the gradient coil driving requirements as well.
This application is related to earlier-filed, commonly assigned, patents and applications of Crooks et al including U.S. Pat. Nos. 4,297,637; 4,318,043; 4,471,305; and pending (now allowed) application Ser. No. 515,177 and now U.S. Pat. No. 4,599,565. The contents of these teferenced related patents and/or patent applications is hereby incorporated by reference.
Magnetic resonance imaging (MRI) is now coming into widespread commercial usage. Nevertheless, there are still many possible areas for improvement. One such area for potential improvement relates to more efficient data acquisition techniques. Another area for potential improvement relates to improvements in the quality of the resulting images.
Multiple section imaging as performed by Crooks et al (see the above-referenced related U.S. patents and patent applications and see also Kumar, Welti, Earnst at J. Mag. Res. 18, 69-83, 1975) utilizes slice selective magnetic gradient pulses which are "on" during each radio frequency pulse (e.g. 90.degree. nutation pulses and 180.degree. nutation pulses) so as to achieve NMR at the Larmor frequency within a selected planar volume without substantially disturbing the spin lattice of adjacent planar volumes. (Each incident radio frequency pulse is typically amplitude modulated by a sinc(t) function so as to select a substantially square edged volume in the spatial domain.)
Slice or other partially selective NMR, in general, has been extensively studied and reported in the literature. See, for example:
"Selective Pulses in NMR Imaging: A Reply To Criticism" by P. Mansfield et al, J. Mag. Res. 33, 261-274 (1979) PA0 "The Solution of The Block Equations in the Presence of a Varying B.sub.1 Field--An Approach to Selective Pulse Analysis" by D. I. Hoult, J. Mag. Res. 35, 69-86 (1979) PA0 "Variations in Slice Shape and Absorption as Artifacts in the Determination of Tissue Parameters in NMR Imaging" by I. R. Young et al, Mag. Res. Med. 2, 355-389 (1985) PA0 "Selective Irradiation Line Scan Techniques for NMR Imaging" by Lawrence E. Crooks, IEEE Trans. N. Sci., NS-27, No. 3, 1239-1244 (June 1980).
After extracting the useful spin echo NMR RF response from a given planar volume, it is allowed to relax to its quiescent alignment with a static z-axis magnetic field while, in the meantime, other planar volumes are similarly selectively defined by suitable magnetic gradient pulses and sinc-modulated RF NMR pulses (with offset frequency spectra) so as to produce the desired NMR spin echo responses from a succession of other planar volumes.
After a sequence of planar volumes have thus been irradiated and their respective NMR responses captured for subsequent analysis, the entire cycle is repeated many times with incrementally increased magnetic gradient pulses along an orthogonal y-axis so as to encode spatial information. Spatial information for the second x-axis dimension is typically encoded by imposing a constant magnetic gradient pulse along the x-axis during each spin echo NMR signal readout. The y-axis phase encoding is changed for each of M NMR cycles so as to provide a linearly increasing progression of y-axis phase encodings (the number of resulting image lines along the y-axis will be equal to the number M of phase encoding cycles of the sequence). A two-dimensional Fourier transformation process is then utilized to obtain the final NMR image (see above referenced U.S. patent application No. 515,117).
This prior Crooks et al technique is depicted at FIG. 2 of the present application. As will be appreciated, for a given y-axis resolution of M lines per image, one must repeat the measurement cycle M times. A given measurement cycle can only be repeated after at least about one T1 interval (often on the order of one second or more).
The TE time interval between the successive spin echoes in a given measurement cycle permits the T2 NMR time constant to be calculated while the TR time interval between repetitions of the measurement cycle within a given complete measurement sequence permits the T1 time constant to be calculated. In this way, one may calculate and display NMR images of the T1 and/or T2 time constants for each elemental voxel of nuclei. Typically, the easiest NMR image to obtain simply displays the measured NMR response signal intensity for each elemental voxel within the image field thus avoiding the extra time and complexity required to calculate T1 and/or T2 NMR time constants.
Although a sinc shaped envelope is used in the time domain for the RF nutation pulses in an attempt to achieve a "square" shaped frequency spectrum, such perfection is not absolutely achievable in practice. For example, a practical system cannot utilize RF nutation pulses with a true complete sinc shaped envelope since that would imply an infinitely long time duration for the RF nutation pulse. Accordingly, a somewhat truncated version of the sinc function is actually utilized and this itself will give rise to some slope at the edges of the frequency spectrum envelope which results (as will be apparent to those in the art from a consideration of the Fourier transform relationships between the time domain and frequency domain). The actual region in which magnetization is nutated depends only to first order on the frequency content of the RF pulse. The actual distribution of magnetization depends on non-linear interaction of spin magnetization with the time varying (modulated) RF magnetic field. Detailed techniques for analyzing this interaction are given in the references by Mansfield and Hoult. Furthermore, where a succession of slice selective RF nutation pulses are utilized in each measurement cycle (as shown in FIG. 2), the net resulting useful signal actually depends upon accurate spatial conjunction of each of the individual slice selection processes associated with each of the RF nutation pulses.
For these and perhaps various other reasons, it has heretofore been necessary to assume certain minimum sized "gaps" between the selected slices for which NMR image data is collected within a given single data acquisition sequence (e.g. M repetitions of the FIG. 2 basic measurement cycle). In effect, this gap has resulted because of an inability to achieve accurate NMR nutation effects out to the extreme edges of a desired planar volume or slice through the object under test. Rather, at the outer edges of any such attempted slice selection geometry, one does not truly achieve either 90.degree. or 180.degree. nutation but, rather, something much less. It is only in the central portion of the selected slice volume that one actually obtains the desired 90.degree. and 180.degree. nuclei nutation. Accordingly, either (a) the nuclei towards the edges of the selected slices do not contribute at all to the measured NMR responses (in which case they truly represent a non-imaged "gap") or, (b) to the extent they may produce some measurable NMR responses, those responses are not the actually desired response and hence tend to contaminate and degrade the desired NMR imaging process.